I really don't think so. The Mac has a different history and use case and is heavily used within Apple, among other important places.
The incentive for Apple to do this would be what: do extract a little more revenue from their smallest-volume OS? As you say, part of its use case is the fact that you can just download and run code on it. That's a small and decreasing percentage of users, but those users are important too.
I'm glad there's no sideloading on my phone and ipad (of course I can always run my own code when I want to). It means my parents have stable platforms to use. And I'm extra glad that restriction doesn't apply to my mac.
I like how the design language of the watch is influencing the ios and macos devices. Unlike the stupid experiments on the phone, there isn't a lot of room on the watch for "flat" interfaces: space is at a premium and contrast is an important factor.
I'm also glad that for almost the first time new features are rolling out to i devices and macos at the same time. Usually it was mobile first, then grudgingly macos.